Ammonia oxidation processes to produce nitric acid and hydrogen cyanide using precious metal gauze catalysts are well established. In the manufacture of nitric acid, ammonia is oxidised with air to nitric oxide, while in the manufacture of hydrogen cyanide a mixture of ammonia and methane (often as natural gas) is oxidised with air. Both are typically performed by contacting the gases with a precious metal catalyst often in the form of a gauze prepared from platinum or a platinum-alloy. In both processes, the gas mixture is passed at an elevated temperature (e.g. 800 to 1000° C.) over a catalyst to effect the oxidation.
Recently, improvements in base-metal catalysts have offered alternatives to precious metals with the added benefit of producing low levels of nitrous oxide, which is a potent greenhouse gas.
Particulate ammonia oxidation catalysts based on cobalt mixed metal oxides, such as those described in WO98/28073, have proven capable of performing this task with excellent efficiency and the desired selectivity. The catalytic oxidation of ammonia is very fast, so the particulate beds are typically less than 500 mm thick. In such beds, maintaining a uniform distribution of pellets and hence of gas flow through the bed can be difficult. This arises from various factors, including plant vibration and variable thermal profiles across the catalyst, but predominantly is due to the effect of the substantial change in diameter of the reaction vessels, as they change temperature from ambient to operating conditions above 850° C. and then back again on shut down. The frequency of such shut downs, can be relatively high, with a consequent effect arising, potentially cumulatively, on each cycle. The effect of severe bed thinning can occur in particular around the periphery of the catalyst bed where the resulting bypass of ammonia can reduce process efficiency below an economic level, as well as increasing emissions of greenhouse gases and, in severe cases, produce an explosion hazard.
This problem has been successfully solved using special catalyst support baskets for instance as described in WO03/011448. These counteract the detrimental effects of expansion and contraction of the catalyst bed structure. They are however, complex to fabricate and need to be carefully sealed within the reaction vessel to avoid gaps that can themselves promote gas bypass.
A stable, thin bed of pelletised catalyst, capable of achieving the high selectivity for the desired oxidation product, without either excessive pressure drop across it or bypass around or through it, is therefore extremely desirable.
Moreover, with metal oxide-based ammonia oxidation catalysts it has been found that careful attention to the start-up process, also known as light-off, is required to ensure the catalytic reaction is established at or very close to the top of the bed to reduce risk of quenching resulting from the relatively low thermal conductivity of the catalyst compared with precious metal gauzes. A similar quenching effect may also be observed where the feed gases contain appreciable amounts of sulphur compounds, which can poison cobalt-based catalysts. While this may be solved in some circumstances using hybrid arrangements of precious metal gauzes in combination with particulate ammonia oxidation and/or nitrous oxide abatement catalysts as described for example in WO04/096703 and WO04/096702, there remains a need to improve the metal oxide catalyst light-off ability and resistance to poisoning.